server : use pytest-xdist for server tests (#28298)

* server : use pytest-xdist for server tests

This commit adds pytest-xdist to the server tests. This is pytest
plugin that distributes test execution across multiple CPU cores.

Assisted-by: pi:llama.cpp/qwen3.8-27B

Refs: https://github.com/ggml-org/llama.cpp/pull/26734#issuecomment-5220707042

* remove server_base_port and BASE_PORT

* use worksteal and pytest builting tmp_path

tools/server/tests/unit/test_compat_anthropic.py

@@ -21,7 +21,6 @@ def create_server():
     global server
     server = ServerPreset.tinyllama2()
     server.model_alias = "tinyllama-2-anthropic"
-    server.server_port = 8082
     server.n_slots = 1
     server.n_ctx = 8192
     server.n_batch = 2048
@@ -34,7 +33,6 @@ def vision_server():
     server = ServerPreset.tinygemma3()
     server.offline = False  # Allow downloading the model
     server.model_alias = "tinygemma3-anthropic"
-    server.server_port = 8083  # Different port to avoid conflicts
     server.n_slots = 1
     return server
 
@@ -1015,7 +1013,6 @@ def test_anthropic_thinking_with_reasoning_model(stream):
     server.jinja = True
     server.n_ctx = 8192
     server.n_predict = 1024
-    server.server_port = 8084
     server.start(timeout_seconds=600)  # large model needs time to download
 
     if stream: